The invention relates to a terminal screw assembly for the electrical connection of cables or wires, in particular cables and wires provided with ear-shaped cable terminals. In one exemplary embodiment, the terminal screw assembly comprises a housing and a clamping screw which, in the screw in direction, penetrates through a first clamping plate and, when the terminal is closed, through a second clamping plate and, when the terminal is open, remains sunk in a bore of the first clamping plate and opens up the interspace between the first and the second clamping plate, the clamping screw being screwconnectable in the bore of the first clamping plate, and the second clamping plate comprising a bore having a screw thread, and wherein the shank of the clamping screw is reduced to a small diameter in a region which is adjacent to the screw head and the axial length of which at least corresponds to the axial length of the engagement of the first clamping plate in threads of the clamping screw.
Terminal screw assemblies of this kind are, for example, used as connecting terminals for switches, electrical apparatus, such as motors, or also for sockets. Such a terminal assembly has been disclosed in U.S. Pat. No. 4,611,876. According to that U.S. patent, the second clamping plate is arranged in the housing in a fixed manner, whereas the first clamping plate is displaceable in the axial direction. In order, in this case, to limit the screw-out movement of the clamping screw and to retain the clamping screw in the bore of the first clamping plate, in the open position of the terminal, the clamping screw is guided in a cage which is connected to the first clamping plate. This cage is guided to be non-rotatable and to be axially displaceable in the housing, with clearance. Depending on the position of the terminal, said cage, therefore, either falls with the first clamping plate on to the second clamping plate, or it moves away therefrom. Thus, when the terminal is open, the free spacing between the first and the second clamping plate is not ensured, with the result that, when the terminal is open, the insertion of the cable or the cable terminal between the first and the second clamping plate is rendered more difficult. This is the case, in particular, when the terminal screw is not readily accessible or during overhead installation. In addition, the design of the first clamping plate comprising a cage retaining the clamping screw is complex.
The object of the invention is, therefore, to develop a terminal screw of the kind mentioned at the outset in such a way that the insertion of cables, in particular cables having ear-shaped cable terminals, is substantially simplified. In order to meet this object, the design according to the invention essentially comprises that the first clamping plate is non-displaceable and non-rotatable in the axial direction of the clamping screw, and the second clamping plate is guided to be non-rotatable and, when the terminal is open to the maximum, to be displaceable, in the axial direction of the clamping screw, out of its position, and that the second clamping plate is held by a slight resistance (for example, by friction) in the maximum open terminal position, in which position the clamping screw, when completely screwed through the first clamping plate, engages the internal screw thread of the second clamping plate, the opening path of the clamping screw being defined by its stopping against a part of the wall of the housing.
As a result of the fact that the first clamping plate is designed to be axially non-displaceable and non-rotatable, the limit stop for the opening path of the clamping screw can, in a simple manner, be formed by a part of the wall of the housing, without requiring additional components. The opening path of the clamping screw is, in this regard, dimensioned such that the clamping screw, in the open position, remains in the bore of the first clamping plate and does not project beyond the clamping face of the first clamping plate. As a result of the fact that the second clamping plate, which is non-rotatable and displaceable in the axial direction of the clamping screw, is held in the maximum open position of the terminal by a resistance, this maximum open position is ensured and the insertion of the cable or cable terminal is simplified in this position of the terminal screw. It is now no longer possible for the second clamping plate to drop towards the first clamping plate, as a result of the force of gravity, thereby obstructing the insertion of the cable or the cable terminal. As a result of the selection of the dimensions such that the clamping screw, when completely screwed through the first clamping plate, engages the thread of the second clamping plate, it is possible to close the terminal while overcoming the resistance which holds the second clamping plate in the maximum open position of the terminal.
This resistance can, in a simple manner, be a friction resistance which is adequate to hold the second clamping plate in its maximum open position, which is defined by a limit stop on the housing, but which can be overcome without difficulty during displacement of the clamping plate.
According to a preferred embodiment of the invention, the bore in the first clamping plate is designed without an internal screw thread, and the first clamping plate is provided with projections which project inwards and form parts of threads and engage in a thread of the clamping screw, with a clearance in parts. This provides the advantage that, although the projections permit a screwing insertion of the clamping screw until the second clamping plate is reached, they do provide a clearance such that the clamping screw can readily engage the first thread of the internal screw thread of the second clamping plate. During continued screwing into the first clamping plate, said projections are completely released, as a result of the fact that the shank of the clamping screw is designed to be reduced to a smaller diameter in the region of the screw head, such that the complete screw-fastening of the second clamping plate is not obstructed. According to the invention, the second clamping plate may, for example, have at least one tongue which is angled off in the axial direction of the clamping screw, perpendicular relative to the clamping face, which tongue is guided between guiding faces of the housing, said guiding faces being disposed in the axial direction of the clamping screw, and the tongue is clamped with friction contact between these guiding faces, and these guiding faces may have projections which clamp the tongues in a friction-tight manner, in order to reduce the friction to a minimum. According to a preferred embodiment, it is, however, also possible to provide the first clamping plate with resilient claws, the ends of which are curved in the direction of the axis of the clamping screw and lap over the second clamping plate and hold it back resiliently, in the maximum open position of the terminal. As a result hereof, the second clamping plate would be held more reliably in its stopping position on the housing and the resistance during the displacement of said clamping plate is more readily adjusted.
Said projections may be formed, for example, on projections along the thread-less bore wall of the first clamping plate or by a ridge which is cut by the clamping screw. According to a preferred embodiment of the invention, the arrangement is, however, such that the projections which engage in a thread of the clamping screw are provided on a resilient disc, preferably of spring steel sheet, which is connected to the first clamping plate and is provided with a bore, the diameter of which is smaller than the outside diameter and larger than the core diameter of the screw thread of the clamping screw, from which radiate a number of radial slits which divide the inner region of the disc into tongues which engage in at least one thread of the clamping screw. Preferably, this resilient disc is attached to a surface or the first clamping plate, for example by spot welding. According to the invention, the resilient disc may, in this regard, be secured to the clamping face of the first clamping plate, and the resilient claws which, in the complete open position, lap over the second clamping plate, may be integral with the resilient disc. This resilient disc can, preferably, be composed of beryllium bronze which has a good electric conductivity and a high elasticity. It is, in this regard, appropriate that the edge of the bore in the first clamping plate is rounded off or provided with a conical depression on that side which faces the elastic disc, in order to permit the tongues formed on the disc to deflect into the thread of the clamping screw.
According to the invention, it is expedient that the spacing between the first clamping plate and the limit stop, which defines the opening movement of the clamping screw and is formed by a part of the wall of the housing, is dimensioned such that, in the stopping position, the clamping screw sinks into the bore of the first clamping plate but does not project, at least not substantially, beyond the clamping face of the first clamping plate. In this manner, the clamping screw can be screwed into the projections of the bore of the first clamping plate until the clamping screw engages the second clamping plate. According to the invention, the housing in the axis of the clamping screw may have an opening for penetration by a screwdriver, the smallest dimension of which opening is smaller than the diameter of the head of the clamping screw, such that the edge of said opening itself forms the limit stop for the opening path of the clamping screw. In addition, the advantage is provided that the clamping screw cannot be lost and is protected by this counterbore.
According to the invention, the axial length of that part of the clamping screw which is provided with a thread is greater than the axial spacing between the clamping face of the second clamping plate and the ridge or inwardly projecting projections or the resilient disc of the first clamping plate, when the terminal is open to a maximum, such that the clamping screw can reliably be screwed through the opening in the first clamping plate until it engages the second clamping plate, in the maximum open position of the terminal. It is, in this regard, expedient that the free end of the shank of the screw is designed as a thread-less centering pin, such that it is ensured that the thread of the second clamping plate is engaged. The bore of the second clamping plate is provided with a normal screw thread.
The clearance between the rigid projections or the ridge of the first clamping plate and the thread of the clamping screw is expediently dimensioned such that the projections which engage in a thread of the screw, or the engaging ridge, are disposed on an inside diameter which is greater than the core diameter of the screw by at least 30%, preferably 40 to 70%, of the difference between the core diameter and outside diameter of the screw.
The invention will be described hereinafter in more detail with reference to exemplified embodiments which are diagrammatically illustrated in the drawings.